Many wind instruments, especially woodwinds, utilize pads set into cups to cover tone holes in the instrument which, in turn, control the pitch produced by the instrument. Flutes, for example, typically have seventeen pads in cups which cover seventeen tone holes.
Pads can be made of many materials, both natural and synthetic. The most common pad is an assembly comprised of felt backed by cardboard and wrapped in collagen membrane. Cork is also a common material for a natural pad.
Each pad must be installed in its respective cup so that it makes the most complete seal possible in its closed position over the tone hole rim. A great deal of effort can be required to properly align a pad, especially the large sizes, with the tone hole rim over which it is positioned. Smaller pads, such as those found on piccolos, are typically floated into position on a bed of shellac or other adhesive. Good success has been reported floating in pads on a silicon adhesive.
Larger pads are generally not flat enough to be floated into place. For the larger pads highly skilled padders utilize paper or plastic shims of about 0.001" to about 0.020" thick to adjust the alignment of the pad. The addition of a shim behind part or all of the pad has the effect of pushing forward the pad so that it moves closer to the tone hole rim. After shimming the pad, the padder takes a feeler gauge made of a thin material like cigarette paper, inserts the feeler between the pad and the tone hole rim, and closes the pad against the rim. The padder then pulls gently on the feeler to test the tension. Insufficient tension is an indication that the pad is not sealing well in a particular location. The padder repeats this test around the entire rim. If the pad is not covering completely, the key is disassembled from the instrument and reshimmed. A great deal of experience is required to accurately use the feeler and to know just how thick a shim to insert in which location. This shimming process is tedious and can take several days.
Moreover, the natural pads made from felt, collagen and cardboard are subject to change over time and with environmental changes. The membranes are porous and allow moisture to get absorbed by the felt. The membranes contain fibers of different densities which differentially absorb moisture. When the pad absorbs moisture and swells, it does so differentially, causing degradation of the seal between the pad and the tone hole rim.
Pads not only affect how well an instrument plays (related to how good the seal is), but how much "popping noise" is emitted by the instrument action of the pad hitting or slapping against the tone hole rim. Popping noise is undesirable, and the harder the pad material, the more popping noise. A soft pad will produce less noise, but may also feel too "spongy" to the musician, who determines when to stop pushing a key down by the feeling of positive contact that occurs when the pad hits the tone hole rim. A hard or firm pad surface is desirable to give the right feel and sound but such a surface may not conform well to produce the proper sealing between the pad and tone hole. A softer pad improves conformability for better sealing, but typically produces a spongy feel, may take aesthetically undesirable deep impressions from the tone hole rim, and is prone to stick to the tone hole rim.
The inside of a wind instrument is a relatively harsh environment due to corrosive saliva, residual sugars and acids from food eaten, moisture, and rapid temperature changes. This environment attacks the pads and can result in periods of only several months between expensive pad changes. Existing natural pads and some synthetic pads are very difficult to clean once in place as they are not compatible with liquids and solvents.
Great effort has been expended in developing new pads which attempt to address one or more of the above-mentioned problems. Some of these pads still require complicated shimming, such as by selectively bending a metal collar. Others rely on synthetic materials to deform sufficiently to establish a proper seal. Some of these pads are too soft, however, and produce a spongy feel during use. Others are too hard, resulting in excess popping noise and reverberation.